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Radon (chemical symbol Rn) is a chemical element with atomic number 86 and belongs to the noble gases in group 18 of the periodic table. It is colorless, odorless, and tasteless, and it is radioactive. Radon occurs naturally as a gas produced by the radioactive decay of uranium and thorium in rocks and soil.

Radon is formed in the decay chains of heavy elements such as uranium-238 and thorium-232. The most common natural isotope, radon-222, has a half-life of about 3.8 days, while other isotopes such as radon-220 and radon-219 have much shorter lives. Because it is a gas, radon can migrate through soil and enter buildings, where it can accumulate to higher concentrations.

As a noble gas, radon is chemically inert, but its radioactivity makes it hazardous. It is denser than air and can seep through foundations and gaps, contributing to indoor radiation exposure. Radon decays into a series of short-lived progeny that can attach to aerosols and, when inhaled, deposit in the lungs. Long-term exposure increases the risk of lung cancer, and radon is a major source of natural radiation exposure worldwide. Risk is amplified by cigarette smoking and by higher indoor concentrations.

Measurements and mitigation are widely used in public health and building safety. Radon levels are tested with detectors such as alpha track detectors, charcoal canisters, and continuous radon monitors. If elevated levels are found, mitigation measures include sealing entry routes, increasing ventilation, and installing sub-slab depressurization systems.

Historically, radon was identified around 1900 as the emanation from radium and was named for this emanation. Today it is mainly discussed in the contexts of public health, environmental science, and occupational safety.